This invention relates to acoustic projectors, and more particularly to underwater, small, low weight, depth independent, coreless electromagnetic acoustic projector systems capable of producing coherent broadband high power low frequency acoustic pressure waves in fluids.
A transducer is a device that converts energy from one form to another. Underwater acoustic transducers generally take acoustic wave inputs from the water and convert the wave to an electrical output signal, known as hydrophones, or take an electrical signal input and produce an acoustic wave output, known as projectors. Both hydrophones and projectors are widely used in sonar systems used for submarine and surface ship applications. Projectors are generally well known and take many forms, including mechanical, chemical and electrical. Acoustic projection systems operating at or below 1 kHz have low propagation loss and thus are an area of study for long-range sonar for both military and oceanographic applications. Size and weight are two of the biggest challenges faced when designing high power (on the order of 209 dB re 1 xcexcPa at 1 m), low frequency projectors. Some additional concerns are environmental impacts and cost.
One method commonly used to generate high power, low frequency acoustic waves is to detonate an explosive charge such as TNT or HBX material. However, explosive charges produce unwanted extraneous effects such as light flashes or bubbles and the sound is incoherent and not able to be precisely reproduced. Additionally, explosives are extremely hazardous to handle and may also leave chemicals in the water from incomplete combustion, which can adversely impact the environment. Also, these systems are good for one time only and become expensive if a large series of acoustic wave outputs are desired.
One system capable of generating frequencies on the order of 10 Hz, which is in the frequency range of interest, is disclosed in U.S. Pat. No. 4,189,026, wherein igniting a combustible gas mixture produces the acoustic wave. However, this device requires fuel and air to be supplied either from the surface or from tanks in order to achieve combustion. Another gas combustion device disclosed in U.S. Pat. No. 5,229,977, obviates the need for a surface supply or tanks by relying on electrolysis of water to produce the needed gas combustion mixture. These devices introduce undesirable extraneous effects such as heat, light and bubbles into the water and are not precisely repeatable.
Another class of projectors that overcome some of these undesirable effects utilizes electromechanical drivers. One such device is depicted in U.S. Pat. No. 5,268,879. This device uses electromagnet assemblies that include a magnetically permeable core and solenoids. However, core saturation places limits on the output power that is obtainable from these types of projectors. This can be overcome by increasing the size of the core to obtain the necessary output, but the size and weight can become unwieldy quickly.
Piezoelectric materials offer some advantages for sonar use. However, these ceramic materials are fragile and expensive. Additionally, performance of a sonar transducer made of piezoelectric material is limited by the inherent energy density of the driver material and its efficiency, especially for applications with size restrictions that require very high source levels and/or very low frequencies. Source level is also limited by the material volume change limitations that are characteristically small with piezoelectric. These materials have been optimized over the years and no further large improvements are expected. It is expected that further improvements must come from new transducer types or from improved materials.
Accordingly, there is a need for a reusable, rugged, small size, high power, low frequency projector system that is capable of producing coherent broadband, high power, low frequency acoustic waves.
An underwater low frequency acoustic projector system comprises two spiral wound wires that are flexibly joined to permit movement. In accordance with one aspect of the present invention the wires are insulated and when energized the coiled wires are repelled from each other.
In accordance with another aspect of the invention the spiral wound wires are separately encapsulated and then joined flexibly in a parallel orientation contiguous with one another. When energized the encapsulated spiral wound wires are repelled from one another and then come back together when not energized. This imparts a pressure wave to the water.
In accordance with still another aspect of the invention the spiral wound wire pairs may be formed into an array that may be used in conjunction with electronic beam forming systems to increase directivity and directional power output.
Still other aspects of the present invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment. It is possible to modify the invention in obvious respects without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.